DENSITIES OF LITHIUM, SODIUM, POTASSIUM, ETC.
117
hoped that the colloidal tungstic acid can be prepared in the near future and its stability studied. Conclusions. T h e electrical conductivity appeared to be the best property to use in studying the stability or instability of these salts. Knorre followed the transformation of a paratungstate by titration. But this changes the composition of the mixtures and may lead to doubtful results. T h e refractometer was not applicable because of the great dilution of the solutions. T h e results so far obtained are chiefly valuable in indicating the y a k of change of the tungstates in question. Assuming that the transformarion products may be new salts or mixtures of several known salts, it is obvious that these solutions will only possess properties characteristic of the salts taken when freshly made up, and certainly not after prolonged boiling. T h e effect of a transformation has been overlooked especially in solubility determinations. For example, Marignac stated that the solubility of ammonium paratungstate was still slowly increasing even at the end of a year. His final result is obviously too high as are also probably those in the case of the other paratungstates.' T h e conductivity data so far obtained are insufficient to decide whether the transformation consists in the addition or rearrangement of water, a hydrolytic dissociation, or a decomposition of one salt into a mixture of two others, as suggested by Knorre. T h e data a t hand are submitted for the benefit of others at work upon similar problems; and as rapidly as possible further conductivity data will be collected upon metatungstic acid and its salts. T h e possibility of these salts being partly hydrogen or acid salts is being considered, and it is hoped that before long the tungstates can be classified as simply as the phosphates. CNIVERSITY OF PENNSYLVANIA, 1906.
[CONTRIBUTION F R O M THE CHEMICAL
LABORATORY O F HARVARD COLLEGE.]
DENSITIES OF LITHIUM, SODIUM, POTASSIUM, RUBIDIUM AND CAESIUM BY
THEODORE WILLIAM RICHARDSA N D FRANCIS NEWTON BRINK Received December
21, 1906
The densities of the alkali metals are of especial interest for several reasons. I n the first place, they include the lowest of all the metallic densities; in the next place, the highest members of this series of elements have the largest atomic volumes of all solid substances, each of the metals forming a cusp in the atomic volume curve. Again, these are the most compressible o f , metals, and have the largest coefficients of expansion: two properties which are closely concerned with volume, and, therefore, with density. Cf. Taylor, loc. cit.
118
THEODORE W M . R I C H A R D S A N D FRAXCIS N . B R I N K
Many determinations of the densities of these metals have been made in the past by various investigators,' but oddly enough there appears to be no published statement of any investigation concerning the density of lithium since it was first investigated by Bunsen fifty years ago. This is the more remarkable because lithium, being the lightest of all metals, would naturally be supposed to h a r e been the object of considerable interest in this respect. T h e attention of one of the present writers was called to this matter some years ago on account of a series of investigations upon the significance of atomic volume and atomic compressibility. In the course of the determinations of the compressibility of lithium, it became important to know the precise density of the sample in hand, and accordingly, this was determined with great care by the method of floating. T h e determination led to a surprising result, indicating a density nearly ten per cent. less than that given by Bunsen. So large an error naturally led to the suspicion that Bunsen's lithium was impure, the admixture of almost any substance i n so light a material tending to cause an excessive density. This inference led to a further suspicion concerning the results for most of the other alkali metals, a s these results are by no means consistent with one another, and usually not accompanied with any descriptions of analytical tests, or proofs of the purity of the material used. Obviously the whole matter needed a systematic and careful revision, as indeed is the case with most of the recorded data concerning densities. T h e investigation was undertaken not merely a s a matter of curiosity with regard to these light metals, but rather primarily to afford accurate data for theoretical inference concerning the significance of changing atomic volume. Method of Defermirtatioiz.-In the following investigation, the method of floating, devised in the first place by Davy in 1808 for determining the specific gravity of sodium, u a s used in the cases of all the alkali metals with the exception of caesium. H e mixed a heavy and light liquid until he obtained a mixture in which pieces of sodium neither rose nor sank;. but remained suspended within the body of the liquid. I t only remained then to determine t h e specific gravity of the liquid. I n this method there are two sources of error. First, because t h e oxides of these metals are more dense than the metals themselves, the presence of any oxide raises the specific gravity. Second, any action of the metals on the solution by which a gas is evolved has the opposite effect, because of adhering bubbles. These tendencies tend to counterbalance one another, but it is by no means safe to allow them to come into play n-ith that expectation. i For details see Landolt-P,ornsteio, Meyerhoffer, Physiclie- Chernische Tabellen, j d ed., 1). 224.
DENSITIES OF LITHIUM, SODIGM, POTASSIUM, ETC.
119
I t became, therefore, a matter of importance to find two liquids of suitable densities which could be used for this purpose. I n the case of sodium, Davy used oil of sassafras and kerosene. Experimentation, however, with oil of sassafras showed that its use was entirely out of the question, since it gave with sodium a bluish green precipitate and the sodium itself became covered with a layer of the same substance. T h e oil gradually became dark brown in color and finally opaque. Several other liquids were tried and found wanting. T h e liquids finally decided upon for this work were liquid paraffin oil and carbon tetrachloride. T h e paraffin oil was obtained from rectified Pennsylvania kerosene, rejecting all boiling below 150' or above 250'. These liquids were used in all the determinations made by the floating method. They were distilled over and kept in contact with sodium until all moisture was removed and there was evidence of no further action of the sodium on the liquids. I n the case of the carbon tetrachloride, this required some time, but finally a point was reached where the sodium could be kept in the liquid for a sufficient length of time without appreciably losing its bright metallic lustre. T h e determinations were carried out at zoo in a carefully regulated thermostat. A small piece of the metal after being pressed, and cut under oil until wholly bright, was placed in a large test-tube containing the paraffin oil, and carbon tetrachloride was added until the metal rose and remained suspended within the body of the liquid. Fresh pieces of metal were added from time to time so as to find the specific gravity of a fresh piece rather than that of one which had been in the liquid for some time. T h e pieces were examined with especial care for minute bubbles, whose combined effect may be very considerable. T h e liquid, of course, should be well stirred to insure uniform density throughout. Owing to the volatility of the carbon tetrachloride, the Ostwald pycnometers, used for determining the density of the mixture, were fitted with ground glass caps. All weights were reduced to the vacuum standard. Because it was impossible to find a liquid in which lithium would not float, weighed pieces of platinum wire were wound around weighed pieces of lithium, and each of these different combinations was investigated as if it had been a piece of heavier metal. Caesium was investigated in a pycnometer for solids because no heavy liquid seems to exist upon which it will not act. Paraffin oil was used as the liquid to be displaced. Other details will be discussed under the headings of the respective metals. Lifhium.--The lithium used was the purest preparation of a German firm. Spectroscopic tests revealed no sodium or potassium, and qualitative analysis discovered only iron and aluminum. For quantitative
T H E O D O R E WM. R I C H A R D S A N D F R A N C I S N . B R I N K
I20
analysis 0.41 gram was dissolved in alcohol. After oxidation it yielded gram of mixed ferric and aluminic oxides, of which 0.0008 was shown by titration to be ferric oxide, leaving o.oo20 gram of aluminic oxide. In another determination 0.44 gram of lithium yielded o.ooz2 gram of aluminic oxide, in satisfactory agreement with the first. Thus the metal possessed the following composition.
0.0028
Lithium, by difference Aluminum Iron
99.59 per cent. 0 . 2 7 per cent. 0.14 per cent. -100.00
T h e lithium retained its bright metallic lustre during the "floating" there being no apparent action between the liquid and the metal during the time required to make a determination. T h e metal was kept under paraffin oil, and the weight of each piece was obtained in the following manner. After pressing in a pellet-machine and cutting, the piece was wrapped with a weighed platinum wire. T h e clinging oil was then removed by dipping the metal into successive portions of naphtha, and the naphtha was allowed to evaporate in a current of hydrogen. From out of the tube containing hydrogen the bit of lithium with its platinum wire was quickly dropped into a weighed vessel containing paraffin oil, the gain in weight being quickly noted. T h e specific volume (X) was calculated as follows : Let W equal the weight of the lithium, W', the weight of the platinum, 21.5, the density of the platinum, aud D the average observed density of the system. Then
T h e three pycnometers held respectively 8.2269, 8.7258, 8.7544 grams of water at 20°, and weighed in air. All weights were reduced to the vacuum standard. DENSITY OF LITHIUN. Exp.
I
Weight of Lithium and Platinum gram.
0.1436
2
3
4 5 6 7 S
w e i ht of J a t i num gram.
0.0718
,
"
0.1301
